Because they must meet widely differing construction requirements, platens for office equipment represent components which are relatively complicated and thus expensive. Traditionally, a platen is built up of a roller-shaped core consisting of a metal or plastic tube to which a bearing axle or at least axle stubs made of metal are coaxially attached. A cover, usually of rubber, is placed on the core and, in order to obtain sufficient surface exactitude and the correct surface properties, this cover is ground. To produce the desired movement of the platen or its ratchet, the bearing axle often has a so-called line-advance wheel/notched wheel combination mounted rigidly on the axle; a line-advance occurs when a pawl on the machine grips the line-advance wheel and rolls the platen by at least a partial line. The platen is held in the position to which the line-advance pawl acting on the line-advance wheel moves it, by a spring-mounted ratchet stud fixed to the machine body and gripping the ratchet. Such line-advance wheel/notched wheel combinations may be present at both ends of the platen so that it can be rolled both forward and backward. To roll the platen by hand, a platen turning wheel is fixed to at least one side of the bearing axle.
In order to obtain an acceptable acoustic performance from platens of the above type, the tube forming the core is either subdivided by stays or filled with various materials. In order to simplify the complicated construction of the platen, or at least its production, it has already been suggested in German Patent 15 61 267 to spray a bearing axle with plastic in such a way as to produce a roller-shaped core, on one end of which a line-advance wheel has been molded. The surface of the roller-shaped base is then covered with a rubber cover and its interior is divided into chambers by dividers running along the inside of the bearing axle.
While such a platen is much more simply and easily constructed than those of the traditional form described above, it still requires a rubber cover in order to produce an acceptable print background.
From German Published, Non-examined Patent Application DE 36 07 415, a platen for office equipment consisting of an axle with a platen body made of polyurethane resin mounted on it is known. The platen body is formed by the integral foam method in such a manner that the innermost foam layer gradually merges into the outer, compact surface layer, with no obvious divisions. With the corresponding setting of the material, the platen can supposedly also be used as a pressure roller in typewriters and similar office equipment.
Such platens do have surface properties which are roughly similar to those of platens with a rubber cover. However, the polyurethane resin used does not allow parts subject to wear, such as line-advance wheels, ratchets, etc. to be molded in one piece with the platen. In addition, this material does not lend itself to applications in areas of heavy or continuous use, since its properties would lead one to expect a destruction of its surface.